The production of chlorine and caustic soda by electrolysis of aqueous solutions of sodium chloride (hereinafter defined as brine) is one of the most important industrial processes. Chlorine, in fact, is the raw material necessary for obtaining a large variety of solvents, chemical intermediates and plastic materials, such as perchloroethylene, propylene oxide, polyvinylchloride and polyurethane.
Chlor-alkali electrolysis is currently carried out resorting to three different technologies, that is diaphragm, mercury cathode and membrane. The membrane technology has been developed in recent years and is currently used for the construction of new plants. However, great part of the worldwide production of chlorine and caustic soda is still obtained by the diaphragm and mercury technologies, which experienced a slow evolution with time in terms of energy saving, reliability of operation and control of the pollution due to possible release of the fibers used for producing the diaphragm or mercury leaks. This continuous improvement in fact made less interesting under an economical point of view the replacement of existing diaphragm or mercury plants with the modern membrane cells.
In particular, as concerns diaphragm cells, which are the object of the present invention, their structure is essentially made of three parts: a cover, a base on which the anodes are fixed and a cathode provided with internally hollow elements with a rather flat section, known as fingers, interleaved with the anodes.
The base structure is clearly illustrated in U.S. Pat. No. 3,591,483. It preferably comprises a conductive sheet, such as a copper plate, provided with holes, to which the anodes are fixed. The side of the plate facing the anodes is protected by a rubber sheet or preferably a thin sheet of titanium.
The anodes may be in the form of a box, as described in U.S. Pat. No. 3,591,483. However, in a more advanced solution, as described in U.S. Pat. No. 3,674,676, the anodes comprise two opposed movable surfaces supported by flexible means which permit their expansions with the minimization of the anode-cathode fingers distance and the consequent reduction of the cell voltage, that is the energy consumption.
The cathode structure is still today the one described in U.S. Pat. No. 3,390,072. It comprises a hollow box (without cover and base), the external wall of which is made of four carbon steel plates welded along their vertical edges. The box is further provided with an internal wall having welded thereto the fingers made of a perforated sheet or a metal mesh, covered by a porous diaphragm. The geometry of the connections between the external, internal walls and fingers has been optimized as described in DE 4117521A1, which specifies the dimensions of the various parts allowing for minimizing the corrosive action of the cathode on the carbon steel. The porous diaphragm deposited onto the fingers is made of a mixture containing fibers of asbestos or other inert materials such as zirconium oxide, and a polymeric material. The mixture, in a suitable aqueous suspension, is deposited by vacuum filtering. The polymeric material provides for a binding function obtained by subjecting the cathode, with the diaphragm deposited onto its fingers, to a thermal treatment at 250-350.degree. C. In a suitable oven. The proper temperature and necessary time are selected depending on the polymeric material used. Suitable materials are polymers with different degrees of fluorinations, such as polyvinylidenfluoride, ethylenechlorotrifluoroethylene copolymers, polytetrafluoroethylene.
In order to improve the current distribution to the fingers, the thickness of the external wall must be suitably selected. The aforementioned U.S. Pat. No. 3,390,072 describes the use of one or more copper sheets applied to the external wall to avoid using excessively thick carbon steel plates. These copper sheets may be applied by arc welding or explosion bonding. This second method, although much more expensive, is commonly preferred as it ensures a homogeneous electrical contact over all the interface between copper and carbon steel. In the case of copper sheets applied by arc welding, conversely, the electrical contact is essentially localized on the welding areas. Therefore, in this last case, the copper sheets are less efficient in homogeneously distributing electric current among the various fingers and minimizing the ohmic losses, that is the dispersion of electric energy due to the electrical resistance of the structure.
While the performance of both the cover and the conductive base provided with the anodes is satisfactory, the cathode, as previously illustrated, is negatively affected by rather serious inconveniences, which the present invention intends to overcome, as explained in the following discussion. These inconveniences may be summarized as follows:
a) fractures in the welding areas connecting the plates of the external wall, the internal wall and the cathode fingers. This problem, known in the art, is well depicted on the figure at page 176 of the "Corrosion Data Survey", NACE Editions, 1985. From the figure it is soon clear that certain combinations between caustic soda concentration and temperature cause fractures in the carbon steel parts with internal stresses, such as the weld heads. The figure indicates also that the fractures are eliminated if the carbon steel parts are subjected to a stress-relieving thermal treatment. This treatment, consisting in heating at 600.degree. C. for about one hour, cannot be applied to cathodes of the prior art due to the strong differences between the thermal expansion coefficients of carbon steel and copper, which would cause remarkable distortions. On the other hand, a thermal treatment only on the carbon steel structure would be useless, as the subsequent welding of the copper sheets would again involve internal stresses. This situation imposes limitations of both the concentration of the caustic soda produced at the cathode and of the electrolysis temperature, which reduce but do nor eliminate the risk of fractures.
b) Distortions of the cathode structure and fractures in the welding areas between the copper sheet and the carbon steel walls due to thermal fatigue during the diaphragm stabilization phase at 250-350.degree. C. These problems are also due to the different thermal expansion coefficients of copper and carbon steel, as discussed before. Even if the diaphragm stabilization temperatures are substantially lower than those typical of the stress-relieving treatment, the inconveniences are likewise severe as the most commonly used diaphragms today have an average life of 9-15 months and therefore their preparation, including stabilization, is repeated more than once during the operating lifetime of a cathode.
c) Copper salt pollution of the suspension used for depositing the diaphragm.
As the cathode is totally immersed in the tank containing the suspension and as the suspension contains remarkable quantities of chlorides and is saturated with air, unavoidably both the carbon steel parts and the copper parts are subjected to corrosion. The progressive build-up of copper concentration in the suspension may lead to a decay of the diaphragm quality, in particular of the most valuable ones which are foreseen for a longer operating life.
It is an object of the present invention to provide a novel cathode structure made of detachable parts, which overcomes all the above mentioned prior art drawbacks.